Commercial electric power generation systems utilize the liquid cooling of stator windings to remove heat generated in the stator windings during power generation. Many systems are designed such that a hydrogen gas coolant in the generator is at a pressure higher than the coolant fluid pressure (generally oil or water). This serves to minimize the leakage of liquid into the generator frame if an opening develops in the cooling system. However, even with the pressure difference, a long split in a cooling hose would probably produce a major liquid leak even though hydrogen gas would probably also flow into the cooling liquid.
This reduced coolant pressure design (also called a "reverse pressure design") provides an early warning of incipient liquid leaks by the monitoring of the presence of hydrogen gas in the coolant liquid. While the coolant liquid can be oil, unless otherwise specified for purpose of clarity, water will be referred to hereinafter as the coolant. An indication of stator winding coolant system leaks have been diagnosed by one or more of the following events:
(1) An observation of high hydrogen gas usage in the generator;
(2) An increase in water conductivity values resulting from ion particles carried into the water system by gas leakage into the same system;
(3) Measurements of the quantity of hydrogen gas leaving the water reservoir tank vent line (normally measured by placing a plastic bag over the exit of the coolant reservoir vent line and monitoring the time to fill the bag with hydrogen);
(4) The observation of liquid in the bottom of the generator (either visually or by float valve actuated switch);
(5) For oil-cooled stator windings, an indication that the normal vacuum of 28" of Hg cannot be maintained and thus a gas leak is overpowering the reservoir tank vacuum pump.
Normal generator operation provides a slight gas flow of around 3-10 cubic feet per day from the coolant reservoir vent line. Of course, it is desirable never to plug the coolant reservoir vent line (to avoid the possibility of explosion) and thus available small orifice flow meter devices would not be appropriate.
Gas flow can reverse under a number of different circumstances. When reduced generator loads are encountered, the temperature of the gas and or coolant liquid will decrease. Reductions in the reservoir gas temperature can establish reservoir pressures less than atmospheric. It is desirable that any flow meter be able to tolerate occasional reverse flows of gas.
Higher than normal gas flows can result during electrical load increases and/or increases in the reservoir gas temperature would provide a higher than normal gas flow rate out of the reservoir tank. Occasional higher flowrates need to be tolerated by any flow meter.
Because the gas hydrogen (in many generator systems) is an explosive gas (when mixed with oxygen), no device capable of producing sparks could be used.
It is desirable to be able to provide a visual indication of the leakage rate especially if the leakage monitor signal suggests a potential problem. It is also desirable to provide a flow measurement signal which can be monitored and is capable of providing a visual and/or audible alarm if the flow rate exceeds the desired levels.
One such device meeting a number of the above requirements is described in U.S. Pat. No. 4,440,017 issued to Barton et al. on Apr. 3, 1984 entitled "Hydrogen Leak Monitor for a Turbine-Generator," the subject matter herein incorporated by reference. This reference provides a detailed description of the water cooling circuit and the monitoring of leak detection based upon venting of gas from the water storage reservoir. It is noted that this device requires a solenoid valve, two float level switches, a bypass valve, a flow reservoir and a relief valve rendering the system relatively complex.